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What to look for in today's dryers.

When shopping for a desiccant (dehumidifying) resin dryer, you have three basic configurations to choose from: totally machine-mounted, beside-the-press, and the newer portable or off-the-press systems. Central drying systems - a larger version of beside-the-press designs - are a fourth alternative. Within each of the three basic configurations, you also have a choice between a traditional twin-tower (dual-desiccant-bed) type and the newer multiple-bed units.

Your buying decision will have to balance initial price against operating cost, flexibility, reliability, and value-added features. For instance, if you are looking to control more than just temperature, the microprocessor systems now available also monitor logic functions such as direction of air flow and timing of regeneration. They also handle multiple alarm functions like high/low temperatures and safety warnings when switching beds. Although they may cost extra, you may want to invest in controls that can prevent loss of a batch of material damaged by overheating or that can save you from making bad parts from inadequately dried resin.

UP IN THE AIR

Machine-hopper-mounted dryers are typically the low-cost alternative, with starting prices around $4200 for 25-lb/hr capacity. They are best suited to operations where there are infrequent material changes and floor space is limited. On the other hand, maintenance is inconvenient because someone has to climb up on the press to clean the hopper and filter. Also, the size of the dryer you can select may be limited by the physical size of the molding press and how much weight it can take. Many newer small molding machines have guarding around them that makes an awkward fit for hopper-mounted dryers.

Machine-mounted dryers typically have dual desiccant beds, although multiple-bed units can also be had from a couple of suppliers. All typically have automatic regeneration, providing the resin hopper with a continuous stream of low-dewpoint air.

These basically no-frills dryers usually come with digital temperature control. Energy-saving insulated hoppers are standard from a few suppliers. They typically add another $300-400 to the overall cost of the dryer but they also hold in heat better, which translates into shorter drying times.

DOWN ON THE FLOOR

Beside-the-press dryers with machine-mounted hoppers relieve the press from bearing the weight of the dryer and are more convenient to service. They generally cost a few hundred dollars more than totally machine-mounted units. Starting price for a 25-lb/hr system is about $4600.

These dryers are generally best for processors with longer-running jobs that do not require a lot of changeovers. Floor-space requirements are likely to affect your buying decision, as dual-bed units are generally about 30% smaller than multiple-bed units.

Although automatic regeneration is standard, beside-the-press dryers are typically purchased with full microprocessor control. Another typical standard feature is a thermocouple at the hopper inlet, which ensures true temperature control where it matters most - where the inlet air contacts the material. This sensor placement automatically corrects for any heat that is lost through the 10-12 ft process-air hose. Insulated process hose, generally optional, can reduce heat loss at an added cost of about $200. Casters for mobility are often standard but are sometimes a $200-300 option.

PREFERENCE FOR PORTABLES

Steadily gaming in popularity over the last four years is the portable or off-the-press dryer configuration. Here the dryer, hopper, and conveyance system to the press are all mounted on a common floor frame. Prices are higher because two loading systems are often required - one to load the drying hopper and the other to load the molding machine - but these systems offer flexibility since material can be dried away from the press, then wheeled into position where needed for immediate production start-up.

Automated regeneration is standard. These units are also typically purchased with full microprocessor control. A vacuum box, if not standard, will be required for hook-up to the loading system.

State-of the-art, closed-loop portable drying systems use dry air to convey the material to the machine hopper, thus preventing contamination by moist ambient air. They typically have long-life brushless blowers and cartridge or bag-type filters. Bag filters with blowback cleaning are best suited to regrind or dusty materials. Additional cost for dry-air conveying starts around $2300-2400.

For about half as much extra cost ($1200-1400), compressed-air loading systems use shop air to convey material to molding machines. However, the quality of your dried material then will depend on the quality of your compressed air, which must be free of moisture and oil contamination. Also, when throughputs exceed 50 lb/hr, they will consume a lot of compressed air, making these systems expensive to operate.

A third option is a 110-volt, brush-type vacuum motor connected to a low-inventory "JIT" receiver. Lower prices (starting at $900) are their main advantage. However, these systems convey with ambient air, need maintenance of the motor brushes, and require additional power at the molding machine.

Two other common options on portable units are dirty-filter indicators (costing about 8140) and an after-cooling coil for applications where the process heat is over 250 F. This works on the principle that the cooler the air, the more adsorbent the desiccant. Prices start around $300 for a 25-lb/hr after-cooler.

Base price of a complete portable dryer with closed-loop loading to the press starts at around $7500 for a 25-lb/hr size.

Some suppliers offer a portable drying system with two drying hoppers. While one hopper is in use the other can be used to predry another resin for the next run, permitting changeover on the fly. They generally have separate temperature controllers for each hopper, so different resins can be dried, and cost around $3000 more than their single-hopper counterparts.

THE CENTRAL DRYING OPTION

Central drying systems are becoming popular in operations that make many material changes or sample lots of new materials. You can convert your existing beside-the-press dryers to central operation by using them to feed banks of multiple hoppers. This approach costs up to 25% less than purchasing an individual dryer for each material hopper.

A typical scenario involves three or four hoppers mounted on a common frame with a single beside-the-press dryer supplying dry air. This set-up allows you to have dried material ready to start the next job. Suppliers suggest that such systems have individual temperature controllers and booster heaters for each hopper. While this adds approximately $1000 per hopper, the added flexibility of properly drying different resins, they say, will certainly pay off.

A new central drying system with beside-the-press dryer and four-hopper bank ($8500) starts at about $14,000.

HOW MANY BEDS?

For each of the above configurations, dryers can have either the traditional twin-tower design with dual desiccant beds or the newer option of multiple desiccant beds. The latter can have up to 10 beds, though three or four are adequate for most processors. Most multiple-bed units are rotary type, though Dri-Air Industries offers a stationary version.

Advocates of twin towers say this 40-year-old proven technology is simpler in design and easier to service and maintain than multiple beds. It can also be 10-20% less expensive. On the other hand, rotating multiple-bed dryers have smaller desiccant beds or cartridges that are said to allow more energy-efficient utilization of the desiccant.

Multiple-bed dryers all use closed-loop regeneration, which means that dry process air cools the desiccant without preloading it with moisture from ambient air. Some proponents claim that this allows for up to 25% energy savings compared with twin-tower units that use either static or open-loop cooling. Static cooling is achieved by shutting down regeneration heaters and blowers and allowing the heat to radiate into the atmosphere. This method reportedly takes longer than others. Open-loop cooling blows ambient air across the desiccant. Its limitation is that the desiccant can adsorb moisture from the air.

At least two suppliers (AEC/Whitlock and Universal Dynamics) do offer closed-loop regeneration as an option on their twin-tower dryers, although this is said to be a very infrequent requirement.

Proponents of multiple-bed dryers claim two key benefits: minimization of temperature spikes (particularly crucial when drying materials at less than 160 F) and minimization of dewpoint variations after bed changes. A wider temperature range (140-370 F) has traditionally been claimed for these dryers, whereas twin-tower units were said to have performance limitations in the lower-temperature spectrum.

However, suppliers of twin-tower units say recent advancements in design of desiccant beds, valves, and controls have overcome most of the shortcomings associated with earlier models. Some suppliers of twin-tower dryers say their units now can operate with complete temperature stability in the 160-400 F range, and that use of process precoolers allows them to operate even below that temperature. Design upgrades of many of these dryers are also reported to minimize dewpoint fluctuations when changing beds.

Suppliers of multiple-bed dryers have also traditionally cited advantages such as ease of desiccant replacement, elimination of valves, and location of regeneration heaters outside the desiccant beds. Redesign of many twin-tower dryers reportedly has bridged these advantages. For example, tower changes are now said to be easier and faster due to quick-acting clamps. Better seals are said to maintain low dewpoints. And regeneration heaters are now located outside the desiccant, thus extending desiccant life.

EVERYTHING UNDER CONTROL

Most dryer manufacturers offer two basic controllers: a standard digital temperature controller and an optional microprocessor version. The latter is said to be highly popular. It typically includes a dewpoint monitor, dewpoint alarm, temperature alarm, seven-day timer (for programming automatic start-up and shut-down), regeneration temperature control, and SPI protocol for communication from the dryer to the molding machine.

Microprocessor controls add $1500-2000 to the base price of a 25lb/hr dryer and $10,000-$10,500 to a 100-1b/hr unit.

Some suppliers offer "materialsaver" control features aimed at preventing damage to the material by overheating and saving energy in the bargain. A standard feature on Universal Dynamics' FN Series controllers senses when the machine stops using material (e.g., for a mold change or shut-down) and automatically lowers the drying temperature. Dri-Air offers a similar feature as a $150 option.

Thoreson-McCosh offers a control option for around $400 that can determine if the material is dry by measuring the temperature differential across the hopper. If the material is dry, the controls will reduce the heat to a standby temperature.

At the recent NPE show in Chicago, Conair introduced an optional drying monitor with a probe that measures the temperature of material at six different locations in a drying hopper. It is said to provide the most accurate check on whether material has been dried sufficiently before molding. The DM-1 provides an immediate alarm whenever the temperature readings indicate insufficient drying that could lead to part-quality problems. The unit can monitor up to 32 drying hoppers from a central location and is priced at around $2200-3000.
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Title Annotation:resin dryers in the plastics industry
Author:Sherman, Lilli Manolis
Publication:Plastics Technology
Date:Oct 1, 1997
Words:1787
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